The state-of-the-art in fixed mobile convergence (FMC) is the proposal to use WiFi IEEE 802.11 wireless access, in extension of the broadband fixed network, as an adjunct to cellular access. Several manufacturers have focussed on the implementation of so called Dual Transfer Mode (DTM) handsets, which incorporate both WiFi and cellular forms of wireless technology. There are also several voice and data service models envisaged to take advantage of the increased capacity WiFi can offer, as well as the extended coverage.
One principal aim of the DTM is to extend voice service capabilities to include VoIP from those handsets to use the WiFi access when it is in range, and to revert to normal cellular operation when not. The advantage of WiFi is that it is a relatively cheap technology that does not require a license to operate, and therefore it can have a wide range of ownership, such as Enterprises, residential as well as conventional fixed and cellular operators.
Several VoIP service models are possible. One model that is in standardisation by 3GPP is Unlicensed Mobile Access (UMA), in which WiFi access is controlled by a specialised gateway (UGW) that both controls the WiFi Access Points (infrastructure mode), the particular requirements for mobile hand-off between multiple adjacent access points as the handset moves, as well as providing a conventional appearance of a Base Station Controller (BSC) to the cellular network. In operation, UMA calls may thereby be handed-off between WiFi Access Points, as well as to/from the cellular network. UMA is a solution principally for Cellular Network operators (BSC emulation), although it could be conceivably owned by Enterprises and PTTs, this is less likely. The complete cellular voice service remains hosted by the cellular network and available over UMA using Voice over Internet Protocol (VoIP) between the DTM handset and the gateway (UGW). Cellular data service is also available in a manner that mimics the General Packet Radio Service (GPRS) access network, where the UGW behaves as a Serving GPRS Support Node (SGSN) to the cellular network.
A second model that provides voice service over WiFi combines the VoIP capability in the DTM handset with a Session Initiation Protocol (SIP) Client signalling capability; a SIP proxy server is then the complementary part of the network infrastructure. The SIP server can be owned by an Enterprise, PTT or a cellular operator in several variants of this model. Furthermore the SIP server could be IETF based or 3GPP IP Multimedia Subsystem (IMS) based. In any event, where the DTM handset is within WiFi coverage, the handset's SIP Client can signal the SIP proxy server to place calls to another similar user, or to the PSTN, PLMN or PBX extensions via media and signalling gateways. The WiFi Access Points are also controlled in a manner similar to (but independent of) UMA by a controller built at least to IEEE 802.11f standards, to provide handset mobility and to provide a seamless in-call hand-off between access points.
The difficulty with this second model though is that there is no standardised method of handing off a call between the WiFi and cellular access networks, although several have been proposed by the industry. One approach is to use the SIP server in the network to emulate the behaviour of an Mobile Switching Centre (MSC) with direct linkage to other real MSCs of the cellular network. This is analogous to the UMA model but taken up a level in the switching hierarchy, from the BSC to the MSC. While this may provide the means of natively signalling within the cellular network between the emulated and real MSCs to control WiFi and cellular hand-off, there are impositions on provisioning the cellular network to accept the new emulated MSC(s) and also the challenge of designing and executing a fairly complicated and non-standard SIP implementation of an MSC. The solution also remains restricted to working very closely with the cellular operator, although it is more readily owned by an Enterprise or PTT than UMA. Consequently this type of solution has not found much momentum in the industry.
A second approach to the second model is to federate the direct link between the SIP proxy server and the cellular network via a custom designed SIP Mobility Gateway (SMG). In this approach the DTM handset takes on the role of determining potential loss of the WiFi coverage and warns the SMG that it will attempt connection via the cellular network. The handset establishes a cellular hand-off call leg to any MSC associated with the geography, notifies the SMG, and the SMG hands-off the WiFi call leg by also signalling to the MSC to get it to join the two legs of the same call. Hand-off to the WiFi network from the cellular network is also possible, which again is initiated by the DTM handset and supported by the SMG. The cellular MSC is instructed by the SMG to hand-off the call to it, but the MSC remains in the call to provide this function and to be ready in case there is a reversion. Other variants are possible too. This second approach is similar to the first approach using a SIP emulated MSC, but since the special functionality is restricted to the SMG the SIP environment is less compromised than before. However, it still relies on the cellular network to provide hand-off control (MSC), and once the cellular network has been involved, even where the DTM handset moves (back) to the WiFi coverage domain, the cellular network remains in call. This could be very problematic in the situation where the handset was between cells served by two MSCs in the cellular network, where several hand-offs could result in entanglement of the call between the same MSCs and SMG repeatedly. For this reason and similar drawbacks of the first approach, this is not favoured.
A third approach is to incorporate the GPRS data access network in the cellular domain to provide a route for SIP Client signalling. The GPRS forms a (secure) signalling route between DTM handset and the SMG, and therefore avoids using specific MSCs or in principle any other particular infrastructure of the cellular domain. Signalling from the handset can use the SIP Client for both WiFi access and GPRS cellular domains back to the same administration's SMG. The advantage this has over the second approach is that hand-off between WiFi and cellular, whether initiated by the handset and/or the SMG, is performed using SIP to establish/release a call leg between the SMG and the handset through the cellular network, which may also pass through intervening PSTN. This approach neatly decouples any particular reliance on the cellular network infrastructure or signalling; the cellular network simply sees incoming call requests or their release enacted by the SMG or the handset. The only drawback of this method is that the SIP over GPRS must be able to be used simultaneously with the circuit based GSM voice which may be active on a call in many calling scenarios, e.g. transfer. While DTM handsets may soon be able to support simultaneous use of GPRS and GSM, the problem is that many cellular network Base Stations (BTS) do not yet have this capability, and which may only be around when cellular operators upgrade to 3G access.
A fourth approach recognises this delay in functionality to meet the present market demand for DTM. As a variant of the second approach it signals from the SMG using ISUP or PRI to the PSTN/PLMN to effectively transfer calls the to/from cellular network when hand-off is required. The DTM handset instructs the SMG when WiFi coverage is being lost (or vice versa) and the SMG establishes or removes a call to the handset via the cellular network. In the case of WiFi to cellular hand-off the DTM handset recognises the incoming call as the hand-off call leg and seamlessly switches over. The reverse process of cellular to WiFi hand-off simply establishes a VoIP call leg with SIP instructing the SMG to perform a call release in the cellular network. This approach avoids the use of GPRS and of using the cellular network to manage hand-off on the MSC (between WiFi and cellular domains), making the SMG in all respects the controlling point. A restriction of this fourth approach is that to call a user of the service, he must be called at a number representing the SMG, and not on his mobile telephone number in the cellular network which would then bypass the SMG. However the user must have and retain a mobile telephone number so that the SMG can effect the call transfer.
This is not too great a disadvantage and simply means there is a special number for the user that works across the WiFI and cellular access. However, where that user is only within cellular coverage without a means to signal the SMG, when placing a call it will be routed by the cellular network without including the SMG in the end to end path which will mean hand-off cellular to WiFi is not possible unless the methods of one of the first three approaches above is also adopted. The calling line identifier (CLI) will also be that of the mobile telephone number, and not the special number working across WiFI and cellular, making returning any such calls problematic.
In all the above approaches the network SIP Proxy server and/or the SMG have the option to be linked to the Home Location Register (HLR) of a cellular network to determine Presence (e.g. availability, location) information for the user, whether they are registered on the WiFi and/or the cellular domains, and to infer the proper forwarding of any call.
The present invention aims to overcome the disadvantages of the above approaches of the second model and provides a means of effecting hand-off within WiFi coverage (multiple access points) and between WiFi and cellular domains.